Runoff ph isn't very useful as a one-time measurement. Better if you measure it from the start. The trend will be useful. The only "official" runoff test I've seen is the NCSU Pour-Thru Method (google for it). When you read about it you'll understand why casual runoff measuring isn't useful (not enough time to equalize to the soil, too much displaced/diluted). The trend of measurements over time can warn you of a building problem before the plant shows symptoms. Using a decent soil probe like Control Wizard Accurate 8 ($60) can be used to verify runoff readings. Also to see how the ph changes as the soil dries.
If your plants are in flower, I'd suggest flushing with pure water or mild nutrients. Usually acidic soil results from salt buildup. That's from feeding too much and/or with not enough runoff. Flush with about 100% runoff. For example, if you grow in a 2.5 to 3gal container, flush with 2-3 gals. You'll get 1.5 to 2 gal runoff. That improve things considerably if you have accumulated salt.
What soil and nutrients do you use? How much nutrients compared to what the label recommends? Have you measured the ppm of your runoff? That can confirm accumulation. For example, when I had this problem it wasn't critical until runoff ppm was over 1250, approaching 1300 (Hanna scale). As long as it was 1150, runoff and soil ph remained acceptable. These numbers could be different for you. But, it's good watch your own numbers and see the correlations.
You could amend 1-2 tbsp dolomite lime (not hydrated lime) per gallon of soil next time. But, that's not going to fix salt accumulation. If you have that problem you need to reduce your nutes and/or get used to flushing occasionally. Some people will cultivate 1-2 tsp dolomite (per gallon of soil) into the topsoil. But, it takes time for dolomite to have an effect. It may not work evenly into the soil. Won't hurt to do that, but the best bet is to flush.
You can also feed with higher ph to help pull the soil higher. But, that too takes a few waterings to affect the soil's ph. If you have salt accumulation it's like a tug of war (you feeding at ph 7.5, trying to pull the 5.5 soil higher).
More details about soil and nutrients would be helpful. But, 95% of the time flushing is the thing to do.
Also, be sure you're letting your soil dry completely (your container should be very light when you lift it). Soil ph rises as it dries. If you water too frequently it's like holding the ph lower, compounding the problem of acidification.
